1. Field of the Invention
The present invention relates to a secondary battery. More particularly, it relates to a structure connecting an electrode assembly with lead elements.
2. Description of the Related Art
A secondary battery can be charged and discharged unlike a primary battery which is incapable of being recharged. A low capacity battery that has only one battery cell is packaged into a pack shape and can be used as a power source for various portable small electronic devices such as cellular phones, laptop computers, and camcorders. A high capacity secondary battery that has several tens of the battery cells can be used as a power source for driving motors such as in a hybrid automobile.
The secondary battery can be fabricated into various shapes, such as cylindrical and prismatic shapes, and includes an electrode assembly (or a jelly roll) comprising a bar of sheet-shaped positive and negative electrodes and an insulator separator interposed between them, the electrodes and separator being wound and inserted into a case. The case includes a cap assembly has an outer terminal to form a battery.
Each of the positive and negative electrodes is provided with a conductive lead element to pass a current generated during a cell operation. The lead elements pass the current generated in the positive and negative electrodes to a positive terminal and a negative terminal.
When the electrode assembly is integrated with the lead elements which connects the electrode assembly to the external terminals, it is hard to output a voltage generated in each part of the electrode assembly, and a potential difference is caused between the area where the electrode assembly is welded to the lead element and the area apart from the welding area. The potential difference degrades the area where the electrode assembly is welded to the lead element and the extent of the degradation becomes greater as the charging and discharging of the battery is repeated. Consequently, the output of the battery drops and the lifespan of the battery is shortened. The phenomena is even worse in a battery for driving a motor which repeats charging and discharging within a short time.
For this reason, multi-tabs are used as the lead elements in a high-power secondary battery.
The multi-tabs are welded to the current collectors of the positive and negative electrodes, which form the electrode assembly, or they form the multi-tabs as part of the current collectors. This decreases the work efficiency because it requires many workers to manufacture the secondary battery.
Also, since the multi-tab-type lead elements are partially connected to the positive and negative electrodes and pass the current generated at the positive and negative electrodes, it is hard to pass a sufficient current, especially in a motor-driving battery that requires a high output power. Thus, the multi-tab-type lead elements are highly likely to degrade the properties of the battery.